Device for extinguishing a discharge tube having a mercury cathode



Jan. 9,1951

. VAN DORSTEN 2,537,333

A T TOR/V! Y Patented Jan. 9, 1951 DEVICE FOR EXTINGUISHING A DISCHARGETUBE HAVING A MERCURY CATHODE Adrianus Cornelis van Dorsten,Eindhoven,Netherlands, assignor, by mesne assignments, to

Hartford National Bank and Trust Company, Hartford, Conn, as trusteeApplication April 19, 1946, Serial No. 663,292 In the Netherlands March31, 1942 Section 1, Public Law 690, August 8, 1946 Patent expires March31, 1962 6 Claims.

This invention relates'to a device for extinguishing a g'asorvapor-filled discharge tube having a mercury cathode, more particularlyfor use in installations designed for high voltages and large powers andfed with direct current.

The invention has for its object to provide an improved extinguishingdevice wherein the extinction is efiected by means of a voltage impulseobtained from a previously charged condenser, which at the moment ofextinction delivers a discharge impulse through the discharge tube and/or on one or more auxiliary electrodes thereof. As a rule this method ofextinction is very suitable for extinguishing tubes having a mercurycathode in conjunction with the fact that during the extremely shorttime available for the extinction the cathode spot can be caused todisappear so that the discharge is interrupted.

However, this method often entails the drawback that the voltage, whichis reapplied to the discharge tube after extinction and is usually muchhigher than the operating voltage across the tube before the extinction,also exists across the parallel branch consisting of theseries-connection of the above-mentioned switching member and theextinguishing condenser. For this reason the extinguishing condensermust be designed for a much higher voltage than is necessary in view ofits function during the period of extinction. This drawback particularlyoccurs when discharge tubes having a mercury cathode are used, in viewof their fundamental fitness for this purpose, for the passage ofcomparatively large currents of the order of magnitude of hundreds ofamperes at very high voltages, for instance of the order of magnitude oftens of kilovolts. Moreover, it may then occur that 'immediately afterextinction the switching member, the condenser and the feed apparatusconnected in parallel therewith have to take up a current impulse whichmay be a multiple of the extinction current, for instance 100 times aslarge. This drawback may be avoided in regard to the feed apparatus byswitching it oiT temporarily, it is true, but this requires anadditional manipulation. Furthermore, this entails the drawback that theextinguishing condenser is discharged to a higher or smaller degree, itscharge being even reversed under certain conditions, and is notavailable again in time for a next extinction. The latter drawback isparticularly objectionable if the discharge tube, by virtue of itsfunction in a definite circuit arrangement, has to be ignited andextinguished periodically.

T improvement awa 9. theini n qn consists in connecting in series withthe extin guishing condenser and the switching member another(auxiliary) condenser, whose capacity in proportion to the capacity ofthe extinguishing condenser is so chosen as to take up or absorbsubstantially the voltage applied to the discharge tube afterextinction. It will be obvious from What has been said above that in thepresent case the expression substantially is to be understood to meansuch a part of the voltage that the above-mentioned drawbacks cannotoccur.

If, for instance, the voltage returning to the discharge tube uponextinction amounts to several kilovolts, which may occur in transmittingenergy by means of direct current having a high voltage, and the voltageavailable for extinction at the extinguishing condenser is of the orderof magnitude of several hundreds of volts, for instance 500 volts, thevoltage applied to the ex tinguishing condenser will at the utmost be ofthe same order of magnitude after extinction. As a rule it may be saidthat the ratio between the capacities of the extinguishing condenser andthe auxiliary condenser amounts to 50 or 100, the ca;- pacity of theextinguishing condenser being determined, of course, by the quantity ofenergy required for the extinction. Although the auxiliary condensermust be insulated against the high voltage this does not constitute adrawback at all in view of the low capacity relatively to that of theextinguishing condenser. On the contrary, this is advantageous incomparison with operation without such an auxiliary condenser in whichcase the comparatively large extinguishing condenser must be insulatedagainst the same high voltage, since in order to obtain the requiredinsulation the volume of the condenser is substantially proportional tothe square of the voltage.

In order to prevent re-ignition of the tube in special cases on accountof oscillation phenomena (instability) of the discharge, which occur if,after extinction, the voltage returning to the tube would remain at thecondensers connected in parallel therewith, it is advisable that theswitch ing member should be open circuited, as soon as the auxiliarycondenser has been charged by the reapplied voltage and charging currenthas sub stantially ceased to flow. Although, in principle, any suitableswitch may be used to constitute the switching member it is advisable touse a con trollable (auxiliary) discharge tube having'a gasorvapor-filling, which tube may be blocked by means of an auxiliaryelectrode if required, since oscillations can be prevented at any rateby the al ac n.-

In many cases the auxiliary condenser, after extinction, will requiresome time to discharge to such a degree that the main tube, afterreignition, can'be extinguished again by causing the rechargedextinguishing condenser to discharge again through the tube by means ofthe switching member through the intermediary of the auxiliarycondenser. If the moments of extinction are suiiiciently spaced apart,no difficulties will be experienced in regard to the extinction.However, it often occurs that the tube must be rapidly ignited andextinguished successively at such a rate that the discharge of theauxiliary condenser takes place too slowly. In such cases the condenserin the device according to the invention is. shunted by a variableresistance, which may or may not be disconnected by switching means andhas such a value as to permit the discharge to take place within anadjustable time. This is of importance more particularly if the maintube must be periodically ignited and extinguished automatically, forinstance 50 times per see. In this case the desired effect can besecured by connecting in parallel with the auxiliary'condenser aresistance having such a value that the discharge of the condenser hassufficiently proceeded within the time of the period. To this end thetime constant RC must be of the same order of magnitude as the durationbetween two succeeding extinctions.

It is to be remarked that owing to the presence of the resistance, afterextinction of the main tube, a high voltage will be set up also at theauxiliary tube, which might unexpectedly produce again an arc dischargein the auxiliary tube. However, when making use of an auxiliary tubehaving a mercury cathode this drawback is avoided with certainty by sucha choice of the resistance value that the current if any, would lie farbelow the minimum current of a mercury cathode. Owing to this,disturbances in the extinguishing process due to a breakdown of theauxiliary discharge tube, cannot occur. When using other switchingmembers, for instance an auxiliary tube having an incandescent cathode,the shunt-resistance should not be switched on before termination of theextinguishing process and interruption oi the parallel branch. In thiscase the voltage set up at the tube through the resistance does notappear before the passage oiE'cu-rrent through the tube has beeninterrupted.

This step is necessary in conjunction with the well known fact that fora gas-discharge tube having an incandescent cathode there is no minimumcurrent strength at which the discharge is interrupted. The step is alsoadvisable with a tube having a mercury cathode if, under the action ofgas pressure and voltage there would not occur an arc discharge but aglow discharge under certain conditions.

The principles of the invention are applicable to circuits for bothalternating voltage and direct voltage.

The circuit according to the invention has proved to be particularlyeffective in switching off large powers at a high direct voltage of sometens of kilovolts.

In switching oil these large direct current powers the best form ofconstruction of the device according to the invention comprises anauxiliary tube having a mercury cathode, since reignition of the tubeafter extinction of the main tube can be avoided with certainty owing tothe extremely short time of sec. in which the cathode spot disappears,even if the "auxiliary condenser is permanently shunted by theresistance. Consequently, oscillation phenomena in the main tube due tocapacity in parallel with the tube can be avoided automatically even atthe highest voltages.

In order that the importance of the invention for switching off directcurrent having high volt- .ages by means of an extinction impulse from,a condenser on a discharge tube connected in parallel with the switchmay be made better understood, it is remarked in this respect that oneof the causes of the fact that in the strong current technique currentshaving such voltages could "not be switched off up till now, so thatcorresponding switching devices were not sold in commerce, resides inthe oscillation problem which could not be avoided hitherto. The use ofan auxiliary condenser according to the invention yields a solutionwhich, in the combination with a mercury cathode auxiliary tube as aswitching member in the manner described, yields a substantially perfectsolution to the problem. When using a resistance having a suitable valuein parallel with the auxiliary icondenser it is also possible, inprinciple and also for the first time to switch off direct currents ofhundreds of amperes having a high voltage even with a periodicity of 50times per sec. The latter has proved to be of much importance ior thecascade circuit described in French Pat. Spec. 847,401 of applicant forconverting direct current having a high voltage into alternating,current having a lower voltage for the transmission of energy over along distance, since in this case the periodical extinction of thedischarge tubes used in conversion plays an important part.

The invention will be :more fully explained by reference to theaccompanying drawing, given by way of example.

The reference number I designates the main discharge tube which must beextinguished periodically, for instance .50 times "per sec. and may, forinstance, be connected to a source of direct voltage of some tens ofilci lovolts through the intermediary of a load. The tube, for example aso-called Ignitron, is periodically ignited in a common manner byapplying a Whit age impulse between the mercury cathode 3 and theignition electrode 5 which consists of semiconductive material and isplunged into the said mercury cathode.

The extinguishing device according to the invention is connected inparallel with the discharge tube, i. e. in series between the cathode :3and the anode '5. It comprises the extinguishing condenser 6 which ischarged with the polarity indicated in the drawing, from -a directcurrent supply '-I.

In series connection with the extinguishing condenser are an auxiliarycondenser -!2 shunted by the resistance 1'3, and an (auxiliary)"discharge tube 8 having a mercury cathode 9 and an anode H] for which atube of the lg-nitron type having an ignition electrode 'll 'is'chosenin the present case. sistance l3 may, in principle, be omitted if noperiodical extinction is required.

The operation of the device referred to above without resistance I3 is a"follows. If the tube 8 is made conductive by means of the ignitionelectrode l1 thereof and it is assumed that at this moment tube 1 isconductive and the corndenser "l2 uncharged, whilst the extinguishingcondenser 6 is charged in the manner referred to, the extinguishingcondenser will partly dis- As has been set out above, the =rechargeabruptly, the. uncharged condenser 12 being in a position to transmitthis impulse to the anode of the tube l, between whose anode and cathodeprevails at this moment the comparatively low operating voltage of sometens of volts. The auxiliary discharge tube 8 conveys current only forthe extremely short time required for charging the condenser 12, Afterthat the current is interrupted. v I r The current impulse thus obtainedfrom the main condenser produces a short voltage drop at the anode ofthe tube i (sometimes a negative voltage), whereby the tube I isextinguished. As is well known a voltage drop during sec. is sufficientto extinguish the cathode spot of a discharge tube having a mercurycathode.

As this moment a voltage is reapplied between cathode and anode of tubel which may attain, say, some tens of kilovolts. The ratio between thecapacities of the condensers 6, I2 is so chosen that this voltage existsmainly across the condenser l2. r

In'the case referred to without resistance I3 a next extinction afterre-ignition of the tube l by means of the ignition electrode 4 cannottake placebefore the condenser 12 has been discharged in some way.Without resistance l3 this is rather impractical if the extinction hasto take place periodically, for instance 50 times per sec; The value ofthe resistance .13 is now so chosen that the product RC is of the orderof magnitude of the duration of one period or smaller, and consequentlythe discharge of the condenser 12, after an extinction, will haveproceeded to a sufficient degree before the next extinction to permit arecurrence thereof.

Owing to the presence of the resistance I3 a high voltage is applied tothe tube 8 after extinction it is true, but on the other hand the valueof the resistance is chosen such that, if the tube 8 should becomeconductive again, the current through the tube 8 would remain far belowthe minimum current of the mercury cathode so that disturbances due toan arc discharge in the tube 8 cannot occur.

After extinction of the tube the extinguishing condenser E has meanwhilebeen charged again from the supply I to the full voltage and afterre-ignition of the tube I a next extinction can take place periodicallyby igniting the tube l periodically.

If the tube 8 i replaced by another switching member, for instance aspark gap, the operation referred to is fundamentally the same. Whenusing a spark gap the voltage of the condenser 6 must generally behigher to take into account the breakdown voltage of a spark gap, ormeans, for instance an inductance coil, must be available for bringingabout the breakdown of the spark gap. Use may also be made of otherswitching elements capable of being switched on and off periodically atthe required speed; however, a discharge tube having a mercury cathodeis to be preferred also on account of its well-nigh inertia-freeoperation. As such any type of discharge tubes having a mercury cathodemay be used. It is advisable to use tubes having a capacitative ignitionelectrode, since the ignition energy required therefor is very low withrespect to the energy required for other discharge tubes. be used as amain discharge tube.

In switching off direct current of 6 amp/ kilovolts by means of thecircuit represented in the drawing the capacity of the condenser l2 curycathode electrode and an anode electrode,

comprising a series circuit connected in parallel with the said tube andbetween said cathode and saidanode and including an auxiliary condenserelement, a switching element and an extinguishing condenser element, andmeans to apply to said extinguishing condenser element an extinguishingpotential for said discharge tube, said auxiliary and extinguishingcondenser elements Such a tube may also having capacity values in aratio at which voltage impulses applied to the said series circuit aresubstantially absorbed by said auxiliary condenser element.

2. A circuit arrangement for extinguishinga mercur vapor discharge tubeincluding a mercury cathode electrode and an anode electrode,

comprising a, series circuit connected in parallel with the said tubeand between said cathode and said anode and including a parallelconnected auxiliary condenser element and resistance ele ment, anextinguishing condenser element and a switching element, and means toapply to said extinguishing condenser element an extinguishing potentialfor said discharge tube, said auxiliary and extinguishing condenserelements having capacity values in a ratio at which voltage impulsesapplied to the said series circuit are substantially absorbed by saidauxiliary condenser element.

3. A circuit arrangement for recurrently extinguishing in apredetermined period of time a mercury vapor discharge tube having amercury cathode electrode and an anode electrode, comprising a seriescircuit connected in parallel with the said tube and between saidcathode and said anode and including a parallel connected auxiliarycondenser element and a resistance element, an extinguishing condenserelement and a switching element, and means to apply to saidextinguishing condenser element an extinguishing potential for saiddischarge tube, said auxiliary and extinguishing condenser elementshaving capacity values in a ratio at which voltage impulses applied tothe said series circuit are substantially absorbed by said auxiliarycondenser element, said parallel connected auxiliary condenser elementand resistance element having a time constant less than about saidpredetermined period of time.

4. A circuit arrangement for recurrently extinguishing in apredetermined period of time a mercury vapor discharge tube including amercury cathode electrode and an anode electrode,

comprising a series circuit connected in parallel ing capacity values ina ratio at which voltage impulses applied to the said series circuit aresubstantially absorbed by said auxiliary eondenser element, saidparallel connected auxiliary condenser element and resistance elementhav ing a time constant less than about said predetermined period oftime, said resistance element having a resistance value at which thesteady state current through said switching element is maintained belowsaid threshold value.

5. A circuit arrangement for extinguishing a mercury vapor dischargetube including a mercury cathode electrode and an anode electrode,comprising a series circuit connected parallel with the said tube andbetween said cathode and said anode and including an auxiliary condenserelement, an extinguishing condenser ele ment and a second gaseousdischarge tube having a control electrode, and means to apply to saidextinguishing condenser element an extinguishing potential for saidfirst named discharge tube, said auxiliary and extinguishing condenserelements having capacity values in a ratio at which voltage impulsesapplied to the said series circuit are substantially absorbed by saidauxiliary condenser element.

6. A circuit arrangement for extinguishing a mercury vapor dischargetube including a mercury cathode electrode and an anode electrode,comprising a series circuit connected parallel with said tube andbetween said cathode and said anode and including a parallel connectedauxiliary condenser element and resistance element, an extinguishingcondenser element and a second gaseous discharge tube having a minimumcurrent carrying capacity, and means to apply to said extinguishingcondenser element an extinguishing potential for said first nameddischarge tube, said auxiliary and extinguishing condenser elementshaving capacity values in a ratio at which voltage impulses applied tothe said series circuit are substantially absorbed by said auxiliarycondenser element, said resistance element having a resistance value atwhich the steady state current through said second discharge tube ismaintained at a value less than the minimum current carrying capacitythereof.

ADRIANUS CORNELIS VAN DORS'IEN.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS

